1. Field of the Invention
The invention relates to a chemical mechanical polishing (CMP) device, and in particular to a wafer polishing head controlled by gas and liquid pressures.
2. Description of the Related Art
Chemical mechanical polishing (CMP) is a global planarization technique. In chemical mechanical polishing, the rear surface of a semiconductor wafer is fixed by a wafer polishing head. Then, the front surface of the semiconductor wafer is pressed against a polishing pad which is installed on a removable platen. When polishing, a chemical agent conducive to CMP is continuously supplied to the platen. By the chemical reaction between the chemical agent and the front surface of the semiconductor wafer and mechanical polishing, the front surface of the semiconductor wafer can be completely planarized.
The wafer polishing head is used to safely and firmly hold the semiconductor wafer without any damage or contamination on the semiconductor wafer. In an early phase, a semiconductor wafer was adhered on a carrier with a material like wax. After polishing, the wax must be completely removed or the semiconductor wafer is contaminated. Currently, a wafer adhering layer is additionally disposed on the bottom of the carrier. Since the wafer adhering layer is made of a porous material, a semiconductor wafer can be firmly held on the carrier by creating a vacuum environment. However, a high-speed rotation could cause the semiconductor wafer slipping during polishing. Therefore, a retaining ring is additionally installed to surround the semiconductor wafer, thereby preventing the semiconductor wafer from slip. The retaining ring must be rigid and uneasily react with any chemical agent. Typically, Delrin and Tecktron are used to make the retaining ring.
FIG. 1A is a cross-sectional view illustrating a wafer polishing head 10. Referring to FIG. 1A, a carrier 12 is a main body of the wafer polishing head 10. A wafer adhering layer 14, is disposed on the bottom of the carrier 12. The rear surface of a semiconductor wafer 16 is firmly held on the wafer adhering layer 14 by creating a vacuum environment during wafer loading. A retaining ring 18 surrounds the carrier 12 and the semiconductor wafer 16, wherein the bottom of the retaining ring 18 must have a lower position than that of the carrier 12, such that the semiconductor wafer 16 can be prevented from damage during polishing. Furthermore, a first pressure chamber 20 is disposed directly above the retaining ring 18. A diaphragm 22 is disposed on the bottom the first pressure chamber 20 and contact the retaining ring 18. When a gas flows into the first pressure chamber 20, the diaphragm 22 is deformed to press again the retaining ring 18, thereby fixing the retaining ring 18. A second pressure chamber 24 is disposed directly on the carrier 12. When a gas flows into the second pressure chamber 24, a force is created to push the semiconductor wafer 16 via the carrier 12.
FIG. 1B is a flow chart illustrating a pressure control of the wafer polishing head 10 of FIG. 1A. In FIG. 1B, a gas source 26 supplies a gas with a fixed pressure value to the first pressure chamber 20. The second pressure chamber 24 and the carrier 12.
During polishing, the retaining ring 18 always contacts the diaphragm 22, resulting in an abrasion therebetween. Under this condition, it is easy to cause the semiconductor wafer 16 slipping if the bottom of the carrier 12 is lower than that of the retaining ring 18. Therefore, the slipping wafer is easily broken. It is necessary to regularly and manually adjust the retaining ring 18 thereby to maintain the bottom of the retaining ring 18 at a lower position than that of the carrier 12 and to prevent the wafer from being broken.
Additionally, the wafer polishing head uses a gas pressure to provide a vertical force to the semiconductor wafer and the polishing pad, thereby alleviating wabble during polishing. However, the gas pressure depends on the stability of the gas source. As a result, it is easy to cause wabble on the semiconductor wafer and the polishing pad, resulting in a poor polishing uniformity. In order to improve the polishing uniformity, the pressures generated by a gas to press the carrier and the retaining ring are also manually adjusted to different proper pressure values even though the gas comes from the same gas source.